1. Field of the Invention
This invention relates to a packet switching system including an ATM (Asynchronous Transfer Mode).
2. Description of the Related Art
A switching system (Fast Reservation Protocol (FRP)) in an ATM or a packet switching system by which a bandwidth necessary for information transfer, that is, a transmission channel having an appropriate frequency is reserved prior to information transfer and, after the bandwidth is reserved, that is, after the necessary bandwidth is allocated, information is transferred in order to avoid overcrowding of the network is proposed as an FRP system in H. Suzuki et al. "Fast Bandwidth Reservation Scheme with Multilink & Multipath Routing in ATM Networks", IEEE INFOCOM 1991. The FRP system is described with reference to FIGS. 1 and 2. Link 0, link 1, link 2, link 3, link 4, link 5, and link 6 are connected in order from terminal x to terminal y as shown in FIG. 1, and for j=0, 1, 2, 3, 4, 5, switch j+1 is interposed between link j and link j+1. When information to be transmitted is produced, terminal x immediately sends out to link 0 a reservation request packet for reserving bandwidth B necessary for information transfer. Since link 0 may also possibly be used also for information transfer between terminal x and any other terminal than terminal y, if requested bandwidth B is not reserved (fails), link 0 returns a reservation failure packet to terminal x. When bandwidth B is reserved (succeeds), link 0 reserves bandwidth B and sends the reservation request packet to link 1. Bandwidth reservation is also attempted by link 1, link 2, link 3, link 4, and link 5 in a similar procedure to that performed by link 0. It is assumed that link 6 is fixed exclusively to terminal 6 and does not require reservation. When the bandwidth reservation gets through at all of the links from link 0 to link 5 so that a bandwidth is reserved from terminal x to terminal y, terminal x sends information to terminal y. On the other hand, if reservation fails even at only one link, a reservation failure packet is returned to terminal x from the link at which bandwidth reservation has failed. When reservation has failed, the bandwidth reservations from terminal x to the link are canceled and the reserved bandwidths are released. The failure corresponds to a so-called loss in a telephone network, and the probability of failure indicates the quality of the network. This failure is called a block, and the probability of failure is called the block rate. FIG. 2 shows an example of a diagram of transmission of control signals and information between transmitting terminal x and receiving terminal y when FRP communication is employed. FIG. 2 shows that the first block occurs at link 1; the second block occurs at link 4; and the third block occurs at link 2; thereafter, bandwidth reservation succeeds at all of links from link 1 to link 5 and information is transmitted.
In this instance, in the FRP system described above, if bandwidth reservation fails even at only one of the links from link 1 to link 4, the information is blocked irrespective of success or failure of reservation at any other link as shown in FIG. 2. Accordingly, the end to end block rate between transmitting and receiving terminals generally increases according to an increase in the number of links. For example, if it is assumed that the block probability at each link is independent of the block probability at any other links and is represented by g and the number of links to pass between the transmitting and receiving terminals is represented by K, the end to end block rate G is given by G=1-(1-g).sup.K because it is a complementary event of the probability (1-g).sup.K at which the reservation simultaneously succeeds at all of the links. For example, when g is 0.01 and K increases successively from 1, then G successively increases to approximately 0.01, 0.02 and 0.03. In a public telephone network scheme, since a value like 0.01 in the example mentioned above or 0.001 is used as g, G is approximated to G=Kg. Accordingly, when a large number of links must be passed, the block rate becomes very high, for example, in long-distance communications, the block rate increases resulting in a problem in that the communication quality is very low.